I have more than a passing interest in VPN software, and have looked at and used many different implementations over the years. I haven't found much to cheer about, which led me to write tappet for my personal use.
I've been reading about Wireguard for the past few weeks, and I really like it so far. It follows through on many of the same goals that I had with tappet, and goes much further in areas important to more widespread adoption. The author, Jason Donenfeld, articulates the project's design goals in this presentation.
Keeping the code small and easy to review was a primary consideration for me (tappet is under a thousand lines of code, not including NaCl). By this measure, Wireguard does an admirable job of staying small at around 15,000 lines including crypto code and tests.
When I wrote tappet, the Noise Protocol did not exist in a usable (or recommended) form. Wireguard's adoption of this framework brings a host of desirable properties that tappet lacks, notably including perfect forward secrecy.
One of my major frustrations with OpenVPN is the extraordinary time it takes to establish a TLS connection on a high-latency link. Very often, when tethered via GPRS, it will retry forever and never succeed. Tappet goes to the other extreme—it requires zero setup for encrypted links (at the expense of perfect forward secrecy). Wireguard restricts its handshake to a single round-trip, which is an entirely acceptable compromise in practice.
Wireguard runs in the kernel, thereby avoiding the need to copy packets in and out of userspace. I didn't care nearly as much about performance. Tappet is fast enough in userspace that it keps up with the fastest link I've tried it on (42.2Mbps DCHSPA+), and I didn't need anything more.
Wireguard accepts multiple peers per interface, while tappet is limited to setting up point-to-point encrypted links. The former is obviously more practical in realistic deployments. (On the other hand, Wireguard is a Layer-3 VPN, while tappet operates at L2 and forwards Ethernet frames instead of IP packets. How much that matters depends on the circumstances.)
I look forward to a time when I can use Wireguard in production.